I cannot be 100% sure on this but what I have seen today when I was looking for this little cutie is that it is not out during the day in El'Dranil Shallows. I spent 2 hours there today and did not find a single one. But around 9:30pm EST i found 4 close to eachother. Again I am unsure if that might be a night time tame but it is a possibility :)

What a pita to find these (as pets - they seem to usually spawn as non-pets). I accidently had some great luck finding my rares as secondaries to Thornclaw Broodlings in Suramar. The Broodlings only spawn in a small(ish) area and the Sandpipers are pretty reliable as secondaries. If you get frustrated elsewhere, give this a try.

Finally caught a rare today. I found it in Azsuna, right above the final 'a' in Narthalas. It had a poor lvl Bandicoon and a poor level Kelp Scuttler for its backup. I was able to take it down with a lvl 25 Nordrassil Wisp, a lvl 17 Nether Faerie Dragon, and a lvl 23 Thornclaw Broodling on my team. The Nether Faerie did most of the work during the fight.

Note: Avoid using a pet with the swarm/flock/etc. ability. The Coastal Sandpiper can have Sandstorm and it will make it difficult to capture.